Can minimum encrypted message length be used to identify public key algorithm or derived some of its features? Encrypting a very small message using public key cryptography always result in larger encrypted messages. Is it possible to deduce the algorithm used, key length, etc just by looking at the minimum encrypted message length?

More specifically, if I have black box system and when I input a small number of characters to encrypt I get the encrypted version back. Can I deduce anything about the algorithm in the black box using the encrypted message length?
For the most common public key encryption mechanisms such as RSA PKCS#1 encryption mechanism, it is indeed possible to deduce the key length from encrypted length.

In addition to encrypted length, you may want to look at encrypted data. In case of RSA observing encrypted data will reveal e.g. modulus, i.e. basically the public key.

This is generally the case with public key encryption: observing encrypted messages eventually allow to learn the public key.



It is not usually not possible to detect what one of of PKCS#1 paddings was used. However, occasionally, the encrypted message is encapsulated so that it conveys more information on the message. Have you been able to examine encrypted bytes produced by the device, to check if there are e.g. some DER encoded header or other additional information?