When examining SSH authentication logs (/var/log/auth.log), you'll often encounter entries like:

Dec 14 16:29:30 app sshd[22781]: Accepted publickey for dev from XXX.XXX.XX.XXX port XXXXX ssh2: RSA SHA256:pO8i...

This SHA256: value represents the fingerprint of the client's public key in base64-encoded SHA256 format. It's a security feature introduced in OpenSSH 6.8+ to help administrators verify and track which keys are being used for authentication.

To manually verify this fingerprint against a known public key:

ssh-keygen -lf ~/.ssh/id_rsa.pub
256 SHA256:pO8i... user@host (RSA)

Or for a specific key file:

ssh-keygen -lf /etc/ssh/ssh_host_rsa_key.pub

The SHA256 fingerprint is calculated from the public key blob (RFC 4253 format) using:

SHA256(public_key_blob)

Example Python code to generate this fingerprint:

import hashlib
import base64

def calculate_sha256_fingerprint(pubkey):
    # Remove key type and comment
    key_data = pubkey.split()[1]
    decoded = base64.b64decode(key_data)
    sha256 = hashlib.sha256(decoded).digest()
    return 'SHA256:' + base64.b64encode(sha256).decode('ascii').rstrip('=')

# Example usage:
pubkey = "ssh-rsa AAAAB3NzaC1yc2E... user@example.com"
print(calculate_sha256_fingerprint(pubkey))

This fingerprint is particularly useful for:

• Verifying that clients are using expected keys
• Detecting unauthorized key changes
• Auditing SSH access patterns

You can extract these fingerprints from logs for monitoring:

grep 'Accepted publickey' /var/log/auth.log | awk -F'SHA256:' '{print $2}' | sort | uniq

Older OpenSSH versions used MD5 fingerprints, which are less secure. The SHA256 format:

• Is more collision-resistant
• Uses base64 instead of hex for compactness
• Is the current recommended standard

To see both formats:

ssh-keygen -lvf ~/.ssh/id_rsa.pub

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When examining SSH authentication logs, you'll frequently encounter entries like this:

Dec 14 16:29:30 app sshd[22781]: Accepted publickey for dev from XXX.XXX.XX.XXX port XXXXX ssh2: RSA SHA256:pO8i...

This SHA256 string represents the fingerprint of the client's public key used for authentication, presented in Base64 format. It serves as a security verification mechanism to help administrators identify and track which specific keys were used for authentication.

The fingerprint is generated using the following process:

# To manually generate from a public key file:
ssh-keygen -lf id_rsa.pub -E sha256

# Output format:
# 2048 SHA256:pO8i... user@host (RSA)

This matches exactly what appears in your auth.log entries. The fingerprint is calculated by:

1. Taking the raw public key data (not the PEM format)
2. Computing SHA256 hash
3. Encoding the hash in Base64

These fingerprints are particularly useful for:

• Key rotation tracking: Identify when old keys are being used after rotation
• Security audits: Correlate authentication attempts with specific keys
• Troubleshooting: Verify which key was actually used during authentication

Here's how to verify if a key in your authorized_keys matches a log entry:

# Extract the public key fingerprint
grep "ssh-rsa AAAA..." ~/.ssh/authorized_keys | ssh-keygen -lf - -E sha256

You can also generate the fingerprint from private keys (useful when you don't have the public key):

ssh-keygen -y -f ~/.ssh/id_rsa | ssh-keygen -lf - -E sha256

For those managing multiple servers, here's a Python script to extract and analyze these fingerprints:

import re
from collections import defaultdict

def parse_auth_log(log_path):
    pattern = r'Accepted publickey for .*? ssh2: RSA SHA256:(.*?)\s'
    key_counts = defaultdict(int)
    
    with open(log_path) as f:
        for line in f:
            match = re.search(pattern, line)
            if match:
                fingerprint = match.group(1)
                key_counts[fingerprint] += 1
    
    return key_counts

This script creates a frequency count of which keys are being used for authentication, helping identify potential security issues.

While these fingerprints are useful for administration:

• They don't replace proper key management procedures
• Frequent rotation of keys is still recommended
• Consider using certificates instead of raw keys for better traceability