verifying a file signature with openssl dgst

I am signing packets in some Java code and I want to verify the signatures on a C server. I want to fork openssl for this purpose (can always use library functions later... when I know openssl can verify the signatures); however, it's failing to do so:

openssl dgst -verify cert.pem -signature file.sha1 file.data

• all it says is "unable to load key file"

The certificate says:

openssl verify cert.pem 

cert.pem: /C=....
error 20 at 0 depth lookup:unable to get local issuer certificate

However, I specifically don't care about verifying the certificate, I want only to verify the signature for a given file!

The output of openssl x509 -in cert.pem -noout -text is:

Certificate:
    Data:
        Version: 1 (0x0)
        Serial Number:
            ...
        Signature Algorithm: sha1WithRSAEncryption
        Issuer: C=...
        Validity
            Not Before: Feb  1 15:22:44 2010 GMT
            Not After : Jun 19 15:22:44 2037 GMT
        Subject: C=...
        Subject Public Key Info:
            Public Key Algorithm: rsaEncryption
            RSA Public Key: (2048 bit)
                Modulus (2048 bit):
                    00:cc:cc:f9:c7:3a:00:0f:07:90:55:d9:fb:a9:fe:
                    ...
                    32:cc:ee:7f:f2:01:c7:35:d2:b5:9b:35:dd:69:76:
                    00:a9
                Exponent: 65537 (0x10001)
    Signature Algorithm: sha1WithRSAEncryption
        39:d6:2c:6b:6a:00:74:b5:81:c2:b8:60:d6:6b:54:11:41:8d:
        ...
        8f:3e:3f:5d:b3:f8:dd:5e

openssl dgst -verify foo.pem expects that foo.pem contains the "raw" public key in PEM format. The raw format is an encoding of a SubjectPublicKeyInfo structure, which can be found within a certificate; but openssl dgst cannot process a complete certificate in one go.

You must first extract the public key from the certificate:

openssl x509 -pubkey -noout -in cert.pem > pubkey.pem

then use the key to verify the signature:

openssl dgst -verify pubkey.pem -signature sigfile datafile