Given keys in ~/.ssh/authorized_keys format, can you determine key strength easily?

ssh-keygen can do the core of the work (generating a fingerprint from a public key), but it will not automatically process a list of multiple keys as is usually found in an authorized_keys file.

Here is a script that splits up the keys, feeds them to ssh-keygen and produces the table you want:

#!/bin/sh

# usage: authkeys-report <authorized_keys-file>    

set -ue

tmp="$(mktemp -t fingerprint-authkeys.XXXXXXXX)"
trap 'rm -f "$tmp"' 0

while read opts key; do
    case "$opts" in
        [0-9]*|ssh-dss|ssh-rsa)
            # not options, first "word" is part of key
            key="$opts $key"
        ;;
    esac
    echo "$key" >$tmp
    set -- $(ssh-keygen -lf "$tmp")
    bits="$1" fingerprint="$2"

    set -- $key # Note: will mangle whitespace in the comment
    case "$1" in
        [0-9]*) # SSH v1 key
            type=rsa1
            shift 3
        ;;
        ssh-rsa|ssh-dss) # SSH v2 key
            type="$1"
            shift 2
        ;;
        *)
            type=unknown
            set --
        ;;
    esac

    printf '%-14s %-9s %s %s\n' "$type" "$bits" "$fingerprint" "$*"
done <$1

ssh-keygen in openssh-7.2 (Currently in Fedora and Ubuntu Xenial at least) supports reading multiple keys from a single file. Therefore running simply

# ssh-keygen -l -f ~/.ssh/authorized_keys
2048 SHA256:xh0IVbI... jakuje@jakuje (RSA)
2048 SHA256:xh0IVbI... jakuje@jakuje (RSA)

results in the desired output.

If you have zsh, you can do this as a one-liner:

while read line ; do ssh-keygen -lf =(echo $line); done < .ssh/authorized_keys