Name of a constant given its value

Note: As of Go 1.4, the String() code below can be auto-generated using Go's new generate feature, combined with the stringer command. See here for more info.

Apart from ANisus' answer, you can do the following.

package main

import "fmt"
import "crypto/tls"

type Ciphersuite uint16

const (
    TLS_RSA_WITH_RC4_128_SHA            = Ciphersuite(tls.TLS_RSA_WITH_RC4_128_SHA)
    TLS_RSA_WITH_3DES_EDE_CBC_SHA       = Ciphersuite(tls.TLS_RSA_WITH_3DES_EDE_CBC_SHA)
    TLS_RSA_WITH_AES_128_CBC_SHA        = Ciphersuite(tls.TLS_RSA_WITH_AES_128_CBC_SHA)
    TLS_RSA_WITH_AES_256_CBC_SHA        = Ciphersuite(tls.TLS_RSA_WITH_AES_256_CBC_SHA)
    TLS_ECDHE_RSA_WITH_RC4_128_SHA      = Ciphersuite(tls.TLS_ECDHE_RSA_WITH_RC4_128_SHA)
    TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA = Ciphersuite(tls.TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA)
    TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA  = Ciphersuite(tls.TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA)
    TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA  = Ciphersuite(tls.TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA)
)

func (cs Ciphersuite) String() string {
    switch cs {
    case TLS_RSA_WITH_RC4_128_SHA:
        return "TLS_RSA_WITH_RC4_128_SHA"
    case TLS_RSA_WITH_3DES_EDE_CBC_SHA:
        return "TLS_RSA_WITH_3DES_EDE_CBC_SHA"
    case TLS_RSA_WITH_AES_128_CBC_SHA:
        return "TLS_RSA_WITH_AES_128_CBC_SHA"
    case TLS_RSA_WITH_AES_256_CBC_SHA:
        return "TLS_RSA_WITH_AES_256_CBC_SHA"
    case TLS_ECDHE_RSA_WITH_RC4_128_SHA:
        return "TLS_ECDHE_RSA_WITH_RC4_128_SHA"
    case TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA:
        return "TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA"
    case TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA:
        return "TLS_ECDHE_RSA_WITH_AES_128_CBC_SHA"
    case TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA:
        return "TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA"
    }
    return "Unknown"
}

func main() {
    cs := TLS_ECDHE_RSA_WITH_3DES_EDE_CBC_SHA
    fmt.Printf("0x%04x = %s\n", uint16(cs), cs)

    cs = TLS_RSA_WITH_RC4_128_SHA
    fmt.Printf("0x%04x = %s\n", uint16(cs), cs)

    cs = TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA
    fmt.Printf("0x%04x = %s\n", uint16(cs), cs)
}

You can test it on the go playground.

I am afraid that is not possible.
The constants are resolved at compile time and there is nothing in the reflect package that allows you to retrieve the name.

What I suggest is creating a map with the names:

var constLookup = map[uint16]string{
    tls.TLS_RSA_WITH_RC4_128_SHA:      `TLS_RSA_WITH_RC4_128_SHA`,
    tls.TLS_RSA_WITH_3DES_EDE_CBC_SHA: `TLS_RSA_WITH_3DES_EDE_CBC_SHA`,
    ...
}