Creating a rsa public key from its modulus and exponent

Question

I want to verify a RSA signature. I have data to verify, the signature and a public key in a form of modulus and exponent. I'd like to do the verification using openssl. Is it possible? I know I can use `openssl rsautl -verify -in sig -inkey key.pem` but I don't know how (using openssl) to create a public key having just it's modulus and exponent. Maybe other ideas how to check this signature (except writing some programs)?

Accepted Answer

In order to generate a RSA public key in PEM format to be used with `openssl`, you can follow these steps.

Create an ASN1 definition file

Modify the following template to include your modulus and exponent

```
# Start with a SEQUENCE
asn1=SEQUENCE:pubkeyinfo

# pubkeyinfo contains an algorithm identifier and the public key wrapped
# in a BIT STRING
[pubkeyinfo]
algorithm=SEQUENCE:rsa_alg
pubkey=BITWRAP,SEQUENCE:rsapubkey

# algorithm ID for RSA is just an OID and a NULL
[rsa_alg]
algorithm=OID:rsaEncryption
parameter=NULL

# Actual public key: modulus and exponent
[rsapubkey]
n=INTEGER:0x%%MODULUS%%

e=INTEGER:0x%%EXPONENT%%
```

Instead of editing, you also may want to script this using sed

```
sed -i "s/%%MODULUS%%/$(xxd -ps -c 256 mymodulus.bin)/" def.asn1
```

Note the `-c 256` should be chosen according to your key length in bytes.

You can use a similar command for the exponent.

Generate your RSA key

Use the following openssl command. This will give you a DER encoded RSA key.

```
openssl asn1parse -genconf def.asn1 -out pubkey.der -noout
```

Then convert it into a PEM key

```
openssl rsa -in pubkey.der -inform der -pubin -out pubkey.pem
```

Verify using your key

You can use either `openssl dgst -verify` or `openssl rsautl -verify`

Creating a rsa public key from its modulus and exponent

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