I need to send credit-card numbers and other information through SFTP. What considerations should I have? Recommendations? Is valid encrypt this numbers? What does PCI say about this?
Regardless the file transfer protocol, I would recommend encrypting files before sending them out. You may use for example pgp encryption, it can be automated.
Since you are using SFTP, there must be some certificate associate with it(which gives us all the information about the validity and level of security/encryption). Mainly there are two things you should check for such operation.
1) The credibility of the certification authority(Who has issued the certificate). Sometimes self signed certificates are also used.Accept such certificates only if you trust the issuer.
2) The encryption technique and key-length used for encryption. Make sure the encryption technique is not obsolete(RSA is the most common and widely used these days) and the key is long enough to make it hard for anyone to crack it(2048 will be good).