(Swift 3,4,5...) Encrypt/Decrypt using PFX to produce -----BEGIN PKCS7----- ... -----END PKCS7-----

First we encrypt using a certificate + public key to produce output such as:

-----BEGIN PKCS7-----
MIIHPwYJKoZIhvcNAQcEoIIHMDCCBywC ...
...
...
-----END PKCS7-----

Chilkat Downloads for the Swift Programming Language MAC OS X (Cocoa) Objective-C/Swift Libs iOS Objective-C/Swift Libs

func chilkatTest() {
    // This example requires the Chilkat API to have been previously unlocked.
    // See Global Unlock Sample for sample code.

    let crypt = CkoCrypt2()!

    // Specify the encryption to be used.
    // "pki" indicates "Public Key Infrastructure" and will create a PKCS7 encrypted (enveloped) message.
    crypt.cryptAlgorithm = "pki"
    crypt.pkcs7CryptAlg = "aes"
    crypt.keyLength = 128
    crypt.oaepHash = "sha256"
    crypt.oaepPadding = true

    // A certificate is needed as the encryption key.
    // Althought the PFX contains the associated private key, we don't need it for encryption.
    // (A certificate usually contains the public key by default.)
    let cert = CkoCert()!
    var success: Bool = cert.loadPfxFile("qa_data/pfx/cert_test123.pfx", password: "test123")
    if success != true {
        print("\(cert.lastErrorText!)")
        return
    }

    // Tell the crypt object to use the certificate.
    crypt.setEncryptCert(cert)

    var toBeEncrypted: String? = "This string is to be encrypted."

    // Get the result in multi-line BASE64 MIME format.
    crypt.encodingMode = "base64_mime"

    var encryptedStr: String? = crypt.encryptStringENC(toBeEncrypted)
    if success != true {
        print("\(crypt.lastErrorText!)")
        return
    }

    // Make a "-----BEGIN PKCS7-----" ... "-----END PKCS7-----" sandwich...
    let sb = CkoStringBuilder()!
    sb.appendLine("-----BEGIN PKCS7-----", crlf: true)
    sb.append(encryptedStr)
    sb.appendLine("-----END PKCS7-----", crlf: true)

    var outStr: String? = sb.getAsString()

    print("\(outStr!)")

    // Sample output:

    // -----BEGIN PKCS7-----
    // MIICXAYJKoZIhvcNAQcDoIICTTCCAkkCAQAxggH0MIIB8AIBADCBrDCBlzELMAkGA1UEBhMCR0Ix
    // GzAZBgNVBAgTEkdyZWF0ZXIgTWFuY2hlc3RlcjEQMA4GA1UEBxMHU2FsZm9yZDEaMBgGA1UEChMR
    // Q09NT0RPIENBIExpbWl0ZWQxPTA7BgNVBAMTNENPTU9ETyBSU0EgQ2xpZW50IEF1dGhlbnRpY2F0
    // aW9uIGFuZCBTZWN1cmUgRW1haWwgQ0ECEB6M1ZwZdZU7LrAIdurulmUwOAYJKoZIhvcNAQEHMCug
    // DzANBglghkgBZQMEAgEFAKEYMBYGCSqGSIb3DQEBCDAJBgUrDgMCGgUABIIBAK/BZG/iXJ8az7zL
    // 8EQ77mc+oDPQ4w1hyytK2ip4djkPVvTfYhcoDQ+G/DBU+urJfrVBi5H9gmpXwYyfKlyUxBVRVEJl
    // V/V5QQi4JmNTFbmgWh5tp9zDS98l6A2Va4Zs0Wy/owGLfvwitlxd1dsfVAV2hmBYS24BMpNcty5/
    // 0atcKYmSou13G78ztTKdMy1tECgZy8kerMsPdDQbSxEZkT3KpQ8C5uEQqYF3bIVaeZzha/Ywieh/
    // tvO0T4aAmeJufwkNdVECmU7kuhnNaVPXknFl7jeibTl6zA/VcJKBKcIYT9FRC7KjdooI8q+jtQ/V
    // k6RP4POaowkFg1QWRPEWeqIwTAYJKoZIhvcNAQcBMB0GCWCGSAFlAwQBAgQQEEFQduqeJqXQXzy4
    // JpkoDoAgdldJDB9zEkpMpgr5/fR2iLvh5kC6BPfhOYjsawBY4Ok=
    // -----END PKCS7-----

    // ----------------------------------------------------------------------------------------
    // Let's Decrypt the above string.

    // Start with what was produced above..
    sb.clear()
    var bCrlf: Bool = true
    sb.appendLine("-----BEGIN PKCS7-----", crlf: bCrlf)
    sb.appendLine("MIICXAYJKoZIhvcNAQcDoIICTTCCAkkCAQAxggH0MIIB8AIBADCBrDCBlzELMAkGA1UEBhMCR0Ix", crlf: bCrlf)
    sb.appendLine("GzAZBgNVBAgTEkdyZWF0ZXIgTWFuY2hlc3RlcjEQMA4GA1UEBxMHU2FsZm9yZDEaMBgGA1UEChMR", crlf: bCrlf)
    sb.appendLine("Q09NT0RPIENBIExpbWl0ZWQxPTA7BgNVBAMTNENPTU9ETyBSU0EgQ2xpZW50IEF1dGhlbnRpY2F0", crlf: bCrlf)
    sb.appendLine("aW9uIGFuZCBTZWN1cmUgRW1haWwgQ0ECEB6M1ZwZdZU7LrAIdurulmUwOAYJKoZIhvcNAQEHMCug", crlf: bCrlf)
    sb.appendLine("DzANBglghkgBZQMEAgEFAKEYMBYGCSqGSIb3DQEBCDAJBgUrDgMCGgUABIIBAK/BZG/iXJ8az7zL", crlf: bCrlf)
    sb.appendLine("8EQ77mc+oDPQ4w1hyytK2ip4djkPVvTfYhcoDQ+G/DBU+urJfrVBi5H9gmpXwYyfKlyUxBVRVEJl", crlf: bCrlf)
    sb.appendLine("V/V5QQi4JmNTFbmgWh5tp9zDS98l6A2Va4Zs0Wy/owGLfvwitlxd1dsfVAV2hmBYS24BMpNcty5/", crlf: bCrlf)
    sb.appendLine("0atcKYmSou13G78ztTKdMy1tECgZy8kerMsPdDQbSxEZkT3KpQ8C5uEQqYF3bIVaeZzha/Ywieh/", crlf: bCrlf)
    sb.appendLine("tvO0T4aAmeJufwkNdVECmU7kuhnNaVPXknFl7jeibTl6zA/VcJKBKcIYT9FRC7KjdooI8q+jtQ/V", crlf: bCrlf)
    sb.appendLine("k6RP4POaowkFg1QWRPEWeqIwTAYJKoZIhvcNAQcBMB0GCWCGSAFlAwQBAgQQEEFQduqeJqXQXzy4", crlf: bCrlf)
    sb.appendLine("JpkoDoAgdldJDB9zEkpMpgr5/fR2iLvh5kC6BPfhOYjsawBY4Ok=", crlf: bCrlf)
    sb.appendLine("-----END PKCS7-----", crlf: bCrlf)

    let decrypt = CkoCrypt2()!
    decrypt.cryptAlgorithm = "pki"

    // Use the same cert + private key from the PFX above.
    // For decryption, we need the private key.  Given that the certificate was loaded from a PFX,
    // we should already have it.
    success = decrypt.setDecryptCert(cert)

    decrypt.encodingMode = "base64"
    var decryptedText: String? = decrypt.decryptStringENC(sb.getBetween("-----BEGIN PKCS7-----", endMark: "-----END PKCS7-----"))

    print("\(decryptedText!)")

}