It’s a write-up about the challenge : Ringzer0CTF - Public key recovery

Challenge 50 - Public key recovery

1. Challenge

—–BEGIN RSA PRIVATE KEY—– MIICXgIBAAKBgQDwkrxVrZ+KCl1cX27SHDI7EfgnFJZ0qTHUD6uEeSoZsiVkcu0/ XOPbz1RtpK7xxpKMSnH6uDc5On1IEw3A127wW4Y3Lqqwcuhgypd3Sf/bH3z4tC25 eqr5gA1sCwSaEw+yBxdnElBNOXxOQsST7aZGDyIUtmpouI1IXqxjrDx2SQIDAQAB AoGBAOwd6PFitnpiz90w4XEhMX/elCOvRjh8M6bCNoKP9W1A9whO8GJHRnDgXio6 /2XXktBU5OfCVJk7uei6or4J9BvXRxQpn1GvOYRwwQa9E54GS0Yu1XxTPtnBlqKZ KRbmVNpv7eZyZfYG+V+/f53cgu6M4U3SE+9VTlggfZ8iSqGBAkEA/XvFz7Nb7mIC qzQpNmpKeN4PBVRJBXqHTj0FcqQ5POZTX6scgE3LrxVKSICmm6ungenPXQrdEQ27 yNQsfASFGQJBAPL2JsjakvTVUIe2JyP99CxF5WuK2e0y6N2sU3n9t0lde9DRFs1r mhbIyIGZ0fIkuwZSOqVGb0K4W1KWypCd8LECQQCRKIIc8R9iIepZVGONb8z57mA3 sw6l/obhfPxTrEvC3js8e+a0atiLiOujHVlLqD8inFxNcd0q2OyCk05uLsBxAkEA vWkRC3z7HExAn8xt7y1Ickt7c7+n7bfGuyphWbVmcpeis0SOVk8QrbqSNhdJCVGB TIhGmBq1GnrHFzffa6b1wQJAR7d8hFRtp7uFx5GFFEpFIJvs/SlnXPvOIBmzBvjU yGglag8za2A8ArHZwA1jXcFPawuJEmeZWo+5/MWp0j+yzQ== —–END RSA PRIVATE KEY—–

Public key = md5(base64 blob only) MD5 2 first bytes are 0x42f5

2. Solution

• Google: how recover public key from private key openssl found https://stackoverflow.com/questions/5244129/use-rsa-private-key-to-generate-public-key

We used openssl to recover the public key from the RSA private key.

openssl rsa -in private.pem -pubout > public.pem

We copy the public key in a file : key.txt and we used the command.

1
2

md5 key.txt
MD5 (public.pem) = 42f51df8b6a2bafa824c179a38066e5d

The 2 first bytes are 0x42f5, so we submited this md5 to find the flag :

FLAG-9869O2dQ43d1r116kfD0Sj5n