Configuring SSH with x509 authentication on IOS devices

Introduction

This document describes how to configure SSH server with use of x509v3 certificates on IOS devices in accordance with standard RFC6187.
The Secure Shell Protocol (SSH) provides mutual authentication, i.e. both client and server are authenticated. Traditionally, the server uses the RSA private and public keypair for authentication. The SSH client computes the checksum of public key and asks the administrator if it is trusted. The administrator should export the public key from router with use of out-of-band method and compare the values. In practice, this is a cumbersome method and often the public key is accepted without verification, which leads to potential risk of man-in-the-middle attacks.
RFC6187 standard is a solution to this concern as it provides similar level of security and user experience to the TLS (Transport Layer Security) protocol commonly used to protect web-based transmissions.

Prerequisites

Requirements

Cisco recommends that you have knowledge of these topics:

• PKI infrastructure

Components Used

The information in this document is based on these software and hardware versions:

• CSR 1000v router running IOS-XE version 16.6.1
• Pragma Fortress SSH client
• Windows Server 2016 OCSP server
• Identity Services Engine version 2.1

The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, ensure that you understand the potential impact of any command.

Configure

Network Diagram

Deployment considerations

• An RFC6187-compatible SSH client is necessary to take advantage of the feature.
  
  
• The feature has been implemented in IOS version 15.5(2)T and IOS-XE version 15.5(2)S.
  
  
• The SSH client and server negotiates supported authentication mechanisms. All of the authenitcation mechanisms previously supported on the device may continue to run concurrently with x509-based authentication mechanisms in order to ensure smooth transition.
  
  
• Administrator may choose to use x509-based authentication method for server only, client only or both. 
  
  
• The IOS server can verify if the certificate presented by the client is not revoked. In order to do that, the database of revoked certificates is consulted upon each connection. This allows for revocation of access without the need to reconfigure other devices, in case, if the private key of the certificate is compromised or if access for a specific user needs to be revoked.
  
  
• Revocation check is optional, but it is highly recommended to have the possibility to deny access based on compromised credentials. Another option is to to perform authorization for the username fetched from certificate on the external Terminal Access Controller Access-Control System (TACACS)  or RADIUS server. In case the certificate is compromised, the account can be disabled on the external server to prevent access with use of that certificate.
  
  
• The authorization of users can be performed by external server or it can be skipped (all users with a valid certificate assumed to have privileges to access device). The former method is used in this example for simplicity.
  
  
• In order to successfully verify the authentication data of the other party, the client and server only need to trust a common Certificate Authority (CA). This means that only the certificate of CA that signed the router certificate needs to be installed on the client device trusted certificate store.
  
  
• The certificate provides information about identity of the other party (Common Name and Subject Alternative Name are typically used for that purpose). The client should compare the hostname or IP address name of the server that was provided as input by the administrator with the identity data available in the presented certificate. It severely limits the opportnities of man-in-the-middle or other impersonation attacks.
  
  

Configurations

Configure AAA parameters. In a basic scenario (without external authorization server), the authorization for the username fetched from certificate can be skipped.

aaa new-model
aaa authorization network CERT none 

 Configure a trustpoint that holds the CA certificate and optionally the router certificate. 

crypto pki trustpoint SSH
 enrollment mode ra
 enrollment url http://10.1.1.2:80/CertSrv/mscep/mscep.dll
 serial-number
 ip-address 10.0.0.1
 subject-name cn=10.0.0.1
 revocation-check ocsp
 ocsp url http://10.1.1.2/ocsp
 rsakeypair SSH 2048
 authorization list CERT
 ! The username has to be fetched from the certificate for accounting and authorization purposes. Multiple options are available.
 authorization username subjectname commonname 

Tip: In case OCSP server is unreachable, the administrator may choose to disallow all access by using revocation-check ocsp configuration or allow access without revocation check using revocation-check ocsp none (not recommended).

Configure allowed authentication mechanisms used during SSH tunnel negotiation.

! Alorithms used to authenticate server
ip ssh server algorithm hostkey x509v3-ssh-rsa ssh-rsa

! Acceptable algorithms used to authenticate the client
ip ssh server algorithm authentication publickey password keyboard

! Acceptable pubkey-based algorithms used to authenticate the client
ip ssh server algorithm publickey x509v3-ssh-rsa ssh-rsa 

Configure the SSH server to use correct certificates in the authentication process.

ip ssh server certificate profile
   ! Certificate used by server    
   server
      trustpoint sign SSH

   ! CA used to authenticate client certificates
   user
      trustpoint verify SSH

(Optional) Integration with TACACS Server 

After the username is fetched from the certificate, the IOS can perform authorization for that username aginst TACACS server. This is especially useful if the TACACS server is already deployed for device administration. 

Note: The IOS SSH server currently does not support authentication method chaining. This means that if the certificates are used to authenticate the user, the TACACS server cannot be used for password authentication. It can only be used for authorization.

Configure TACACS server.

tacacs server ISE
 address ipv4 10.1.1.3
 key cisco123

Configure the authorization list to use the TACACS server.

aaa authorization network ISE group tacacs+

1. Configure ISE (Identity Services Engine). Configuration example can be found at: 

https://www.cisco.com/c/en/us/support/docs/security/identity-services-engine/200208-Configure-ISE-2-0-IOS-TACACS-Authentic.html

2. Configure TACACS profile. Additional parameter cert-application=all needs to be configured in order for the authorization to succeed, navigate to Work Centers > Device Administration > Policy Elements > Results > TACACS profiles > Add.

3. In order to configure policy set, navigate to Work Centers > Device Administration > Device Admin Policy Sets > Add.

Verify

show ip ssh 
SSH Enabled - version 1.99
Authentication methods:publickey,password,keyboard-interactive
Authentication Publickey Algorithms:x509v3-ssh-rsa,ssh-rsa
Hostkey Algorithms:x509v3-ssh-rsa,ssh-rsa
--- output truncated ----

show users 
 Line User Host(s) Idle Location
 1 vty 0 admin1 idle 00:02:37 192.168.1.100

Troubleshoot

These debugs are used to track successful session:

debug ip ssh detail
debug crypto pki transactions
debug crypto pki messages
debug crypto pki validation

Aug 21 20:07:08.717: SSH0: starting SSH control process
! Server identifies itself
Aug 21 20:07:08.717: SSH0: sent protocol version id SSH-1.99-Cisco-1.25
! Client identifies itself
Aug 21 20:07:08.771: SSH0: protocol version id is - SSH-2.0-Pragma FortressCL 5.0.10.766
Aug 21 20:07:08.771: SSH2 0: kexinit sent: kex algo = diffie-hellman-group-exchange-sha1,diffie-hellman-group14-sha1

! Authentication algorithms supported by server
Aug 21 20:07:08.771: SSH2 0: kexinit sent: hostkey algo = x509v3-ssh-rsa,ssh-rsa
Aug 21 20:07:08.772: SSH2 0: kexinit sent: encryption algo = aes128-ctr,aes192-ctr,aes256-ctr
Aug 21 20:07:08.772: SSH2 0: kexinit sent: mac algo = hmac-sha2-256,hmac-sha2-512,hmac-sha1,hmac-sha1-96
Aug 21 20:07:08.772: SSH2 0: SSH2_MSG_KEXINIT sent
Aug 21 20:07:08.915: SSH2 0: SSH2_MSG_KEXINIT received
Aug 21 20:07:08.916: SSH2 0: kex: client->server enc:aes256-ctr mac:hmac-sha1 
Aug 21 20:07:08.916: SSH2 0: kex: server->client enc:aes256-ctr mac:hmac-sha1 

! Client chooses authentication algorithm
Aug 21 20:07:08.916: SSH2 0: Using hostkey algo = x509v3-ssh-rsa
Aug 21 20:07:08.916: SSH2 0: Using kex_algo = diffie-hellman-group-exchange-sha1
Aug 21 20:07:08.917: SSH2 0: Modulus size established : 4096 bits
Aug 21 20:07:08.976: SSH2 0: expecting SSH2_MSG_KEX_DH_GEX_INIT
Aug 21 20:07:09.141: SSH2 0: SSH2_MSG_KEXDH_INIT received

! Server sends certificate associated with trustpoint "SSH"
Aug 21 20:07:09.208: SSH2 0: Sending Server certificate associated with PKI trustpoint "SSH"
Aug 21 20:07:09.208: CRYPTO_PKI: (A003C) Session started - identity selected (SSH)
Aug 21 20:07:09.208: SSH2 0: Got 2 certificate(s) on certificate chain
Aug 21 20:07:09.208: CRYPTO_PKI: Rcvd request to end PKI session A003C.
Aug 21 20:07:09.208: CRYPTO_PKI: PKI session A003C has ended. Freeing all resources.
Aug 21 20:07:09.209: CRYPTO_PKI: unlocked trustpoint SSH, refcount is 0
Aug 21 20:07:09.276: SSH2: kex_derive_keys complete
Aug 21 20:07:09.276: SSH2 0: SSH2_MSG_NEWKEYS sent
Aug 21 20:07:09.276: SSH2 0: waiting for SSH2_MSG_NEWKEYS
Aug 21 20:07:16.927: SSH2 0: SSH2_MSG_NEWKEYS received
Aug 21 20:07:17.177: SSH2 0: Authentications that can continue = publickey,password,keyboard-interactive
Aug 21 20:07:17.225: SSH2 0: Using method = none
Aug 21 20:07:17.226: SSH2 0: Authentications that can continue = publickey,password,keyboard-interactive
Aug 21 20:07:32.305: SSH2 0: Using method = publickey

! Client sends certificate
Aug 21 20:07:32.305: SSH2 0: Received publickey algo = x509v3-ssh-rsa
Aug 21 20:07:32.305: SSH2 0: Verifying certificate for user 'admin1' in SSH2_MSG_USERAUTH_REQUEST
Aug 21 20:07:32.305: SSH2 0: Verifying certificate for user 'admin1'
Aug 21 20:07:32.306: SSH2 0: Received a chain of 2 certificate
Aug 21 20:07:32.308: SSH2 0: Received 0 ocsp-response
Aug 21 20:07:32.308: SSH2 0: Starting PKI session for certificate verification
Aug 21 20:07:32.308: CRYPTO_PKI: (A003D) Session started - identity not specified
Aug 21 20:07:32.309: CRYPTO_PKI: (A003D) Adding peer certificate
Aug 21 20:07:32.310: CRYPTO_PKI: found UPN as admin1@example.com 
Aug 21 20:07:32.310: CRYPTO_PKI: Added x509 peer certificate - (1016) bytes
Aug 21 20:07:32.310: CRYPTO_PKI: (A003D) Adding peer certificate
Aug 21 20:07:32.310: CRYPTO_PKI: Added x509 peer certificate - (879) bytes
Aug 21 20:07:32.311: CRYPTO_PKI: ip-ext-val: IP extension validation not required
Aug 21 20:07:32.311: CRYPTO_PKI: create new ca_req_context type PKI_VERIFY_CHAIN_CONTEXT,ident 31
Aug 21 20:07:32.312: CRYPTO_PKI: (A003D)validation path has 1 certs

Aug 21 20:07:32.312: CRYPTO_PKI: (A003D) Check for identical certs
Aug 21 20:07:32.312: CRYPTO_PKI : (A003D) Validating non-trusted cert
Aug 21 20:07:32.312: CRYPTO_PKI: (A003D) Create a list of suitable trustpoints
Aug 21 20:07:32.312: CRYPTO_PKI: Found a issuer match
Aug 21 20:07:32.312: CRYPTO_PKI: (A003D) Suitable trustpoints are: SSH,
Aug 21 20:07:32.313: CRYPTO_PKI: (A003D) Attempting to validate certificate using SSH policy
Aug 21 20:07:32.313: CRYPTO_PKI: (A003D) Using SSH to validate certificate
Aug 21 20:07:32.313: CRYPTO_PKI: Added 1 certs to trusted chain.
Aug 21 20:07:32.314: CRYPTO_PKI: Prepare session revocation service providers
Aug 21 20:07:32.314: CRYPTO_PKI: Deleting cached key having key id 30
Aug 21 20:07:32.314: CRYPTO_PKI: Attempting to insert the peer's public key into cache
Aug 21 20:07:32.314: CRYPTO_PKI:Peer's public inserted successfully with key id 31
Aug 21 20:07:32.315: CRYPTO_PKI: Expiring peer's cached key with key id 31
Aug 21 20:07:32.315: CRYPTO_PKI: (A003D) Certificate is verified

! Revocation status is checked
Aug 21 20:07:32.315: CRYPTO_PKI: (A003D) Checking certificate revocation
Aug 21 20:07:32.315: OCSP: (A003D) Process OCSP_VALIDATE message
Aug 21 20:07:32.315: CRYPTO_PKI: (A003D)Starting OCSP revocation check
Aug 21 20:07:32.316: CRYPTO_PKI: OCSP server URL is http://10.1.1.2/ocsp
Aug 21 20:07:32.316: CRYPTO_PKI: no responder matching this URL; create one!
Aug 21 20:07:32.316: OCSP: (A003D)OCSP Get Response command
Aug 21 20:07:32.317: CRYPTO_PKI: http connection opened
Aug 21 20:07:32.317: CRYPTO_PKI: OCSP send header size 132
Aug 21 20:07:32.317: CRYPTO_PKI: sending POST /ocsp HTTP/1.0
Host: 10.1.1.2
User-Agent: RSA-Cert-C/2.0
Content-type: application/ocsp-request
Content-length: 312


Aug 21 20:07:32.317: CRYPTO_PKI: OCSP send data size 312
Aug 21 20:07:32.322: OCSP: (A003D)OCSP Parse HTTP Response command
Aug 21 20:07:32.322: OCSP: (A003D)OCSP Validate DER Response command
Aug 21 20:07:32.322: CRYPTO_PKI: OCSP response status - successful.
Aug 21 20:07:32.323: CRYPTO_PKI: Decoding OCSP Response
Aug 21 20:07:32.323: CRYPTO_PKI: OCSP decoded status is GOOD.
Aug 21 20:07:32.323: CRYPTO_PKI: Verifying OCSP Response
Aug 21 20:07:32.325: CRYPTO_PKI: Added 11 certs to trusted chain.
Aug 21 20:07:32.325: ../VIEW_ROOT/cisco.comp/pki_ssl/src/ca/provider/revoke/ocsp/ocsputil.c(547) : E_NOT_FOUND : no matching entry found
Aug 21 20:07:32.325: ../VIEW_ROOT/cisco.comp/pki_ssl/src/ca/provider/revoke/ocsp/ocsputil.c(547) : E_NOT_FOUND : no matching entry found
Aug 21 20:07:32.326: CRYPTO_PKI: (A003D) Validating OCSP responder certificate
Aug 21 20:07:32.327: CRYPTO_PKI: OCSP Responder cert doesn't need rev check
Aug 21 20:07:32.328: CRYPTO_PKI: response signed by a delegated responder
Aug 21 20:07:32.328: CRYPTO_PKI: OCSP Response is verified
Aug 21 20:07:32.328: CRYPTO_PKI: (A003D) OCSP revocation check is complete 0
Aug 21 20:07:32.328: OCSP: destroying OCSP trans element
Aug 21 20:07:32.328: CRYPTO_PKI: Revocation check is complete, 0
Aug 21 20:07:32.328: CRYPTO_PKI: Revocation status = 0
Aug 21 20:07:32.328: CRYPTO_PKI: Remove session revocation service providers
Aug 21 20:07:32.329: CRYPTO_PKI: Remove session revocation service providers
Aug 21 20:07:32.329: CRYPTO_PKI: (A003D) Certificate validated
Aug 21 20:07:32.329: CRYPTO_PKI: Populate AAA auth data
Aug 21 20:07:32.329: CRYPTO_PKI: Selected AAA username: 'admin1'
Aug 21 20:07:32.329: CRYPTO_PKI: Anticipate checking AAA list: 'CERT'
Aug 21 20:07:32.329: CRYPTO_PKI: Checking AAA authorization
Aug 21 20:07:32.329: CRYPTO_PKI_AAA: checking AAA authorization (CERT, admin1, <all>)
Aug 21 20:07:32.329: CRYPTO_PKI_AAA: pre-authorization chain validation status (0x400)
Aug 21 20:07:32.329: CRYPTO_PKI_AAA: post-authorization chain validation status (0x400)
Aug 21 20:07:32.329: CRYPTO_PKI: (A003D)chain cert was anchored to trustpoint SSH, and chain validation result was: CRYPTO_VALID_CERT
Aug 21 20:07:32.329: CRYPTO_PKI: destroying ca_req_context type PKI_VERIFY_CHAIN_CONTEXT,ident 31, ref count 1
Aug 21 20:07:32.330: CRYPTO_PKI: ca_req_context released
Aug 21 20:07:32.330: CRYPTO_PKI: (A003D) Validation TP is SSH
Aug 21 20:07:32.330: CRYPTO_PKI: (A003D) Certificate validation succeeded
Aug 21 20:07:32.330: CRYPTO_PKI: Rcvd request to end PKI session A003D.
Aug 21 20:07:32.330: CRYPTO_PKI: PKI session A003D has ended. Freeing all resources.
Aug 21 20:07:32.395: SSH2 0: Verifying certificate for user 'admin1'
Aug 21 20:07:32.395: SSH2 0: Received a chain of 2 certificate
Aug 21 20:07:32.396: SSH2 0: Received 0 ocsp-response
Aug 21 20:07:32.396: SSH2 0: Starting PKI session for certificate verification
Aug 21 20:07:32.396: CRYPTO_PKI: (A003E) Session started - identity not specified
Aug 21 20:07:32.396: CRYPTO_PKI: (A003E) Adding peer certificate
Aug 21 20:07:32.397: CRYPTO_PKI: found UPN as admin1@example.com 
Aug 21 20:07:32.397: CRYPTO_PKI: Added x509 peer certificate - (1016) bytes
Aug 21 20:07:32.397: CRYPTO_PKI: (A003E) Adding peer certificate
Aug 21 20:07:32.398: CRYPTO_PKI: Added x509 peer certificate - (879) bytes
Aug 21 20:07:32.398: CRYPTO_PKI: ip-ext-val: IP extension validation not required
Aug 21 20:07:32.400: CRYPTO_PKI: create new ca_req_context type PKI_VERIFY_CHAIN_CONTEXT,ident 32
Aug 21 20:07:32.400: CRYPTO_PKI: (A003E)validation path has 1 certs

Aug 21 20:07:32.400: CRYPTO_PKI: (A003E) Check for identical certs
Aug 21 20:07:32.400: CRYPTO_PKI : (A003E) Validating non-trusted cert
Aug 21 20:07:32.401: CRYPTO_PKI: (A003E) Create a list of suitable trustpoints
Aug 21 20:07:32.401: CRYPTO_PKI: Found a issuer match
Aug 21 20:07:32.401: CRYPTO_PKI: (A003E) Suitable trustpoints are: SSH,
Aug 21 20:07:32.401: CRYPTO_PKI: (A003E) Attempting to validate certificate using SSH policy
Aug 21 20:07:32.401: CRYPTO_PKI: (A003E) Using SSH to validate certificate
Aug 21 20:07:32.402: CRYPTO_PKI: Added 1 certs to trusted chain.
Aug 21 20:07:32.402: CRYPTO_PKI: Prepare session revocation service providers
Aug 21 20:07:32.402: CRYPTO_PKI: Deleting cached key having key id 31
Aug 21 20:07:32.403: CRYPTO_PKI: Attempting to insert the peer's public key into cache
Aug 21 20:07:32.403: CRYPTO_PKI:Peer's public inserted successfully with key id 32
Aug 21 20:07:32.404: CRYPTO_PKI: Expiring peer's cached key with key id 32
Aug 21 20:07:32.404: CRYPTO_PKI: (A003E) Certificate is verified
Aug 21 20:07:32.404: CRYPTO_PKI: (A003E) Checking certificate revocation
Aug 21 20:07:32.404: OCSP: (A003E) Process OCSP_VALIDATE message
Aug 21 20:07:32.404: CRYPTO_PKI: (A003E)Starting OCSP revocation check
Aug 21 20:07:32.405: CRYPTO_PKI: OCSP server URL is http://10.1.1.2/ocsp
Aug 21 20:07:32.405: CRYPTO_PKI: no responder matching this URL; create one!
Aug 21 20:07:32.405: OCSP: (A003E)OCSP Get Response command
Aug 21 20:07:32.406: CRYPTO_PKI: http connection opened
Aug 21 20:07:32.406: CRYPTO_PKI: OCSP send header size 132
Aug 21 20:07:32.406: CRYPTO_PKI: sending POST /ocsp HTTP/1.0
Host: 10.1.1.2
User-Agent: RSA-Cert-C/2.0
Content-type: application/ocsp-request
Content-length: 312


Aug 21 20:07:32.406: CRYPTO_PKI: OCSP send data size 312
Aug 21 20:07:32.409: OCSP: (A003E)OCSP Parse HTTP Response command
Aug 21 20:07:32.410: OCSP: (A003E)OCSP Validate DER Response command
Aug 21 20:07:32.410: CRYPTO_PKI: OCSP response status - successful.
Aug 21 20:07:32.410: CRYPTO_PKI: Decoding OCSP Response
Aug 21 20:07:32.411: CRYPTO_PKI: OCSP decoded status is GOOD.
Aug 21 20:07:32.411: CRYPTO_PKI: Verifying OCSP Response
Aug 21 20:07:32.413: CRYPTO_PKI: Added 11 certs to trusted chain.
Aug 21 20:07:32.413: ../VIEW_ROOT/cisco.comp/pki_ssl/src/ca/provider/revoke/ocsp/ocsputil.c(547) : E_NOT_FOUND : no matching entry found
Aug 21 20:07:32.413: ../VIEW_ROOT/cisco.comp/pki_ssl/src/ca/provider/revoke/ocsp/ocsputil.c(547) : E_NOT_FOUND : no matching entry found
Aug 21 20:07:32.414: CRYPTO_PKI: (A003E) Validating OCSP responder certificate
Aug 21 20:07:32.415: CRYPTO_PKI: OCSP Responder cert doesn't need rev check
Aug 21 20:07:32.415: CRYPTO_PKI: response signed by a delegated responder
Aug 21 20:07:32.416: CRYPTO_PKI: OCSP Response is verified
Aug 21 20:07:32.416: CRYPTO_PKI: (A003E) OCSP revocation check is complete 0
Aug 21 20:07:32.416: OCSP: destroying OCSP trans element
Aug 21 20:07:32.416: CRYPTO_PKI: Revocation check is complete, 0
Aug 21 20:07:32.416: CRYPTO_PKI: Revocation status = 0
Aug 21 20:07:32.416: CRYPTO_PKI: Remove session revocation service providers
Aug 21 20:07:32.416: CRYPTO_PKI: Remove session revocation service providers
Aug 21 20:07:32.416: CRYPTO_PKI: (A003E) Certificate validated
Aug 21 20:07:32.417: CRYPTO_PKI: Populate AAA auth data
Aug 21 20:07:32.417: CRYPTO_PKI: Selected AAA username: 'admin1'
Aug 21 20:07:32.417: CRYPTO_PKI: Anticipate checking AAA list: 'CERT'
Aug 21 20:07:32.417: CRYPTO_PKI: Checking AAA authorization
Aug 21 20:07:32.417: CRYPTO_PKI_AAA: checking AAA authorization (CERT, admin1, <all>)
Aug 21 20:07:32.417: CRYPTO_PKI_AAA: pre-authorization chain validation status (0x400)
Aug 21 20:07:32.417: CRYPTO_PKI_AAA: post-authorization chain validation status (0x400)
Aug 21 20:07:32.417: CRYPTO_PKI: (A003E)chain cert was anchored to trustpoint SSH, and chain validation result was: CRYPTO_VALID_CERT
Aug 21 20:07:32.417: CRYPTO_PKI: destroying ca_req_context type PKI_VERIFY_CHAIN_CONTEXT,ident 32, ref count 1
Aug 21 20:07:32.417: CRYPTO_PKI: ca_req_context released
Aug 21 20:07:32.417: CRYPTO_PKI: (A003E) Validation TP is SSH
Aug 21 20:07:32.417: CRYPTO_PKI: (A003E) Certificate validation succeeded
Aug 21 20:07:32.418: CRYPTO_PKI: Rcvd request to end PKI session A003E.
Aug 21 20:07:32.418: CRYPTO_PKI: PKI session A003E has ended. Freeing all resources.
Aug 21 20:07:32.418: SSH2 0: Verifying signature for user 'admin1' in SSH2_MSG_USERAUTH_REQUEST
Aug 21 20:07:32.418: SSH2 0: Received a chain of 2 certificate
Aug 21 20:07:32.418: SSH2 0: Received 0 ocsp-response
Aug 21 20:07:32.418: CRYPTO_PKI: found UPN as admin1@example.com 

! Certificate status verified successfully
Aug 21 20:07:32.419: SSH2 0: Client Signature verification PASSED
Aug 21 20:07:32.419: SSH2 0: Certificate authentication passed for user 'admin1'
Aug 21 20:07:32.419: SSH2 0: authentication successful for admin1
Aug 21 20:07:32.470: SSH2 0: channel open request
Aug 21 20:07:32.521: SSH2 0: pty-req request
Aug 21 20:07:32.521: SSH2 0: setting TTY - requested: height 25, width 80; set: height 25, width 80
Aug 21 20:07:32.570: SSH2 0: shell request
Aug 21 20:07:32.570: SSH2 0: shell message received
Aug 21 20:07:32.570: SSH2 0: starting shell for vty
Aug 21 20:07:32.631: SSH2 0: channel window adjust message received 8

In case certificate for admin1 has been revoked:

Aug 21 19:39:52.081: CRYPTO_PKI: OCSP Response is verified
Aug 21 19:39:52.081: CRYPTO_PKI: (A0024) OCSP revocation check is complete 0
Aug 21 19:39:52.082: OCSP: destroying OCSP trans element
Aug 21 19:39:52.082: CRYPTO_PKI: Revocation check is complete, 0
Aug 21 19:39:52.082: CRYPTO_PKI: Revocation status = 1
Aug 21 19:39:52.082: CRYPTO_PKI: Remove session revocation service providers
Aug 21 19:39:52.082: CRYPTO_PKI: Remove session revocation service providers
Aug 21 19:39:52.082: CRYPTO_PKI: (A0024) Certificate revoked
Aug 21 19:39:52.082: %PKI-3-CERTIFICATE_REVOKED: Certificate chain validation has failed. The certificate (SN: 750000001B78DA4CC0078DEC0700000000001B) is revoked
Aug 21 19:39:52.082: CRYPTO_PKI: (A0024)chain cert was anchored to trustpoint Unknown, and chain validation result was: CRYPTO_CERT_REVOKED
Aug 21 19:39:52.082: CRYPTO_PKI: destroying ca_req_context type PKI_VERIFY_CHAIN_CONTEXT,ident 18, ref count 1
Aug 21 19:39:52.082: CRYPTO_PKI: ca_req_context released
Aug 21 19:39:52.083: CRYPTO_PKI: (A0024) Certificate validation failed

Related information

• PKI configuration guide:
  https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/sec_conn_pki/configuration/15-mt/sec-pki-15-mt-book.html
  
• TACACS on ISE configuration example:
  https://www.cisco.com/c/en/us/support/docs/security/identity-services-engine/200208-Configure-ISE-2-0-IOS-TACACS-Authentic.html

• Technical Support & Documentation - Cisco Systems