I have a keypair. How do I determine the key length?

Question

Using OpenSSL from the command line in Linux, is there some way to examine a key (either public or private) to determine the key size?

There are many different ways depending on the format of the keypair. Were you looking at a specific format? — Zoredache, Oct 27, 2011 at 19:55
As a rule of thumb, the size (in bytes) of a .pem RSA private key is roughly 3/4 of the size of the key length (in bits) - e.g. a 4096-bit key might be roughly 3247 bytes. File sizes do vary though. — mwfearnley, Dec 3, 2019 at 11:34

Paul · Accepted Answer · 2023-05-01 12:11:07Z

130

openssl rsa -in private.key -text -noout

The top line of the output will display the key size.

For example:

Private-Key: (2048 bit)

To view the key size from a certificate:

openssl x509 -in public.pem -text -noout | grep "Public Key"
RSA Public Key: (2048 bit)

Paul

3,1376 gold badges29 silver badges41 bronze badges

answered Oct 27, 2011 at 19:45

Shane Madden

115k13 gold badges185 silver badges254 bronze badges

1

And if he is using DSA, or something else. How about determining the key size from the public key(cert).
– Zoredache
Oct 27, 2011 at 19:51
@Zoredache Yeah - I could have sworn the question said "private key" specifically; either I'm losing it or an edit beat the 5 minute timer.
– Shane Madden
Oct 27, 2011 at 20:13
4

OpenSSL 1.0.1g 7 Apr 2014 format requires | grep "Public-Key"
– Vadzim
Sep 15, 2015 at 7:23

Add a comment |

Adrien P. · Accepted Answer · 2011-10-27 19:50:14Z

22

The first (2048) is the bit length of the key:

 $ ssh-keygen -lf /etc/ssh/rsa_key.pub 
 2048 91:1c:ae:17:16:...

answered Oct 27, 2011 at 19:50

Adrien P.

8094 silver badges9 bronze badges

9

He said 'openssl', not 'openssh'.
– MikeyB
Oct 27, 2011 at 20:46
This would actually work on a private key generated by openssl genrsa. You would probably have to point it directly to the private key file though.
– mwfearnley
Mar 19, 2021 at 14:40

Add a comment |

2 Answers 2