Convert RSA public key to RSA DER

Question

I have id_rsa.pub key generated by ssh-keygen. How can I programmatically convert id_rsa.pub files to RSA DER formatted keys?

Answer 1

If you use ssh-keygen to generate a key:

$ ssh-keygen

Then you can just use openssl to pull out the public key and write it in the DER format like this:

$ openssl rsa -in id_rsa -out pub.der -outform DER -pubout
writing RSA key

You can view the DER output as PEM like this:

$ openssl rsa -in pub.der -inform DER -pubin -text

I don't use Ruby, so I don't know how easy it is to use OpenSSL from Ruby.

Edit: I answered too quickly -- you wrote id_rsa.pub and you may not have the id_rsa itself. Another Stack Overflow question is for the reverse conversion, but the source code found there might help: Convert pem key to ssh-rsa format Once you have PEM you can use openssl to convert the PEM to DER.

Edit, May 2014: Ruby has become my favorite programming language, and the original question (since edited) asked about Ruby. Here is code to read the id_rsa.pub (public key) and write an OpenSSL-generated, DER-formatted public key:

require 'openssl'
require 'base64'

def read_length(s)
    # four bytes, big-endian
    length = s[0..3].unpack('N')[0]
end

def read_integer(s, length)
    # shift all bytes into one integer
    s[4..3 + length].unpack('C*').inject { |n, b| (n << 8) + b }
end

def cut(s, length)
    s[4 + length..-1]
end

def decode_pub(pub)
    # the second field is the Base64 piece needed
    s = Base64.decode64(pub.split[1])

    # first field reading "ssh-rsa" is ignored
    i = read_length(s)
    s = cut(s, i)

    # public exponent e
    i = read_length(s)
    e = read_integer(s, i)
    s = cut(s, i)

    # modulus n
    i = read_length(s)
    n = read_integer(s, i)

    [ e, n ]
end

def make_asn1(e, n)
    # Simple RSA public key in ASN.1
    e0 = OpenSSL::ASN1::Integer.new(e)
    n1 = OpenSSL::ASN1::Integer.new(n)
    OpenSSL::ASN1::Sequence.new([ e0, n1 ])
end

pub = File.read('id_rsa.pub')

asn1 = make_asn1(*decode_pub(pub))

# Let OpenSSL deal with converting from the simple ASN.1
key = OpenSSL::PKey::RSA.new(asn1.to_der)

# Write out the public key in both PEM and DER formats
File.open('id_rsa.pem', 'w') { |f| f.write key.to_pem }
File.open('id_rsa.der', 'w') { |f| f.write key.to_der }

You can check the output with these openssl commands in the shell:

$ openssl rsa -pubin -text -in id_rsa.pem
$ openssl rsa -pubin -text -inform DER -in id_rsa.der

Answer 2

If you only have access to the public key generated by ssh-keygen, and want to convert it to DER format, the following works:

ssh-keygen -f id_rsa.pub -e -m PKCS8 | openssl pkey -pubin -outform DER

This first uses ssh-keygen to convert the key to PKCS8 PEM format, then uses openssl pkey to convert that to DER format.

(This accomplishes the same thing as Jim Flood's answer, but without touching the private key file.)