Decrypt Safari's Form Values file
So... After hours research, I finally found the encryption process.

The password contained in the keychain is NOT the encryption key!
The password has to be derived to obtain the key. The process is as follow: PBKDF2_SHA1(password: passwordFromKeychain, salt: someSalt, iterations: 1000, outputLen: 128)

Then the cipher can be easily decrypted. It is simply AES-CBC-128 with PKCS5Padding and no IV (or IV = [0, ...])

The output will be a binary PLIST which can be read by any program having such capability. Thus, you can know if the decryption process is successful by verifying the first 6 byte are equals to: "bplist".

I wrote a code (JAVA) to do the decryption: https://github.com/Flo354/SafariFormValuesDecryptor