Kotlin Multiplatform Base64 - no extra dependencies. Based on Java's implementation.

Base64.kt

/*
 * Copyright (c) 2012, 2018, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */

import kotlin.math.min

/**
 * This class consists exclusively of static methods for obtaining
 * encoders and decoders for the Base64 encoding scheme. The
 * implementation of this class supports the following types of Base64
 * as specified in
 * [RFC 4648](http://www.ietf.org/rfc/rfc4648.txt) and
 * [RFC 2045](http://www.ietf.org/rfc/rfc2045.txt).
 *
 *
 *  * <a id="basic">**Basic**</a>
 *
 *  Uses "The Base64 Alphabet" as specified in Table 1 of
 * RFC 4648 and RFC 2045 for encoding and decoding operation.
 * The encoder does not add any line feed (line separator)
 * character. The decoder rejects data that contains characters
 * outside the base64 alphabet.
 *
 *  * <a id="url">**URL and Filename safe**</a>
 *
 *  Uses the "URL and Filename safe Base64 Alphabet" as specified
 * in Table 2 of RFC 4648 for encoding and decoding. The
 * encoder does not add any line feed (line separator) character.
 * The decoder rejects data that contains characters outside the
 * base64 alphabet.
 *
 *  * <a id="mime">**MIME**</a>
 *
 *  Uses "The Base64 Alphabet" as specified in Table 1 of
 * RFC 2045 for encoding and decoding operation. The encoded output
 * must be represented in lines of no more than 76 characters each
 * and uses a carriage return `'\r'` followed immediately by
 * a linefeed `'\n'` as the line separator. No line separator
 * is added to the end of the encoded output. All line separators
 * or other characters not found in the base64 alphabet table are
 * ignored in decoding operation.
 *
 *
 *
 *  Unless otherwise noted, passing a `null` argument to a
 * method of this class will cause a [ NullPointerException][java.lang.NullPointerException] to be thrown.
 *
 * @author  Xueming Shen
 * @since   1.8
 */
object Base64 {

    fun ByteArray.encodeToBase64(): String {
        return encoder.encode(this).decodeToString()
    }

    fun String.decodeFromBase64(): ByteArray {
        return decoder.decode(this.encodeToByteArray())
    }

    /**
     * Returns a [Encoder] that encodes using the
     * [Basic](#basic) type base64 encoding scheme.
     *
     * @return  A Base64 encoder.
     */
    val encoder = Encoder(null, -1, true)

    /**
     * Returns a [Decoder] that decodes using the
     * [Basic](#basic) type base64 encoding scheme.
     *
     * @return  A Base64 decoder.
     */
    val decoder = Decoder()

    /**
     * This class implements an encoder for encoding byte data using
     * the Base64 encoding scheme as specified in RFC 4648 and RFC 2045.
     *
     *
     *  Instances of [Encoder] class are safe for use by
     * multiple concurrent threads.
     *
     *
     *  Unless otherwise noted, passing a `null` argument to
     * a method of this class will cause a
     * [NullPointerException][java.lang.NullPointerException] to
     * be thrown.
     *
     * @see Decoder
     *
     * @since   1.8
     */
    class Encoder internal constructor(private val newline: ByteArray?, private val linemax: Int, private val doPadding: Boolean) {
        private fun outLength(srclen: Int): Int {
            var len = 0
            len = if (doPadding) {
                4 * ((srclen + 2) / 3)
            } else {
                val n = srclen % 3
                4 * (srclen / 3) + if (n == 0) 0 else n + 1
            }
            if (linemax > 0) // line separators
                len += (len - 1) / linemax * newline!!.size
            return len
        }

        /**
         * Encodes all bytes from the specified byte array into a newly-allocated
         * byte array using the [Base64] encoding scheme. The returned byte
         * array is of the length of the resulting bytes.
         *
         * @param   src
         * the byte array to encode
         * @return  A newly-allocated byte array containing the resulting
         * encoded bytes.
         */
        fun encode(src: ByteArray): ByteArray {
            val len = outLength(src.size) // dst array size
            val dst = ByteArray(len)
            val ret = encode0(src, 0, src.size, dst)
            return if (ret != dst.size) dst.copyOf(ret) else dst
        }

        private fun encodeBlock(src: ByteArray, sp: Int, sl: Int, dst: ByteArray, dp: Int) {
            var sp0 = sp
            var dp0 = dp
            while (sp0 < sl) {
                val bits: Int = src[sp0++].toInt() and 0xff shl 16 or (
                        src[sp0++].toInt() and 0xff shl 8) or
                        (src[sp0++].toInt() and 0xff)
                dst[dp0++] = toBase64[bits ushr 18 and 0x3f].toByte()
                dst[dp0++] = toBase64[bits ushr 12 and 0x3f].toByte()
                dst[dp0++] = toBase64[bits ushr 6 and 0x3f].toByte()
                dst[dp0++] = toBase64[bits and 0x3f].toByte()
            }
        }

        private fun encode0(src: ByteArray, off: Int, end: Int, dst: ByteArray): Int {
            val base64 = toBase64
            var sp = off
            var slen = (end - off) / 3 * 3
            val sl = off + slen
            if (linemax > 0 && slen > linemax / 4 * 3) slen = linemax / 4 * 3
            var dp = 0
            while (sp < sl) {
                val sl0: Int = min(sp + slen, sl)
                encodeBlock(src, sp, sl0, dst, dp)
                val dlen = (sl0 - sp) / 3 * 4
                dp += dlen
                sp = sl0
                if (dlen == linemax && sp < end) {
                    for (b in newline!!) {
                        dst[dp++] = b
                    }
                }
            }
            if (sp < end) {               // 1 or 2 leftover bytes
                val b0: Int = src[sp++].toInt() and 0xff
                dst[dp++] = base64[b0 shr 2].toByte()
                if (sp == end) {
                    dst[dp++] = base64[b0 shl 4 and 0x3f].toByte()
                    if (doPadding) {
                        dst[dp++] = '='.toByte()
                        dst[dp++] = '='.toByte()
                    }
                } else {
                    val b1: Int = src[sp++].toInt() and 0xff
                    dst[dp++] = base64[b0 shl 4 and 0x3f or (b1 shr 4)].toByte()
                    dst[dp++] = base64[b1 shl 2 and 0x3f].toByte()
                    if (doPadding) {
                        dst[dp++] = '='.toByte()
                    }
                }
            }
            return dp
        }

        companion object {
            /**
             * This array is a lookup table that translates 6-bit positive integer
             * index values into their "Base64 Alphabet" equivalents as specified
             * in "Table 1: The Base64 Alphabet" of RFC 2045 (and RFC 4648).
             */
            val toBase64 = charArrayOf(
                'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M',
                'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z',
                'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm',
                'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z',
                '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', '+', '/'
            )

            /**
             * It's the lookup table for "URL and Filename safe Base64" as specified
             * in Table 2 of the RFC 4648, with the '+' and '/' changed to '-' and
             * '_'. This table is used when BASE64_URL is specified.
             */
            internal val toBase64URL = charArrayOf(
                'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L', 'M',
                'N', 'O', 'P', 'Q', 'R', 'S', 'T', 'U', 'V', 'W', 'X', 'Y', 'Z',
                'a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm',
                'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z',
                '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', '-', '_'
            )
        }
    }

    /**
     * This class implements a decoder for decoding byte data using the
     * Base64 encoding scheme as specified in RFC 4648 and RFC 2045.
     *
     *
     *  The Base64 padding character `'='` is accepted and
     * interpreted as the end of the encoded byte data, but is not
     * required. So if the final unit of the encoded byte data only has
     * two or three Base64 characters (without the corresponding padding
     * character(s) padded), they are decoded as if followed by padding
     * character(s). If there is a padding character present in the
     * final unit, the correct number of padding character(s) must be
     * present, otherwise `IllegalArgumentException` (
     * `IOException` when reading from a Base64 stream) is thrown
     * during decoding.
     *
     *
     *  Instances of [Decoder] class are safe for use by
     * multiple concurrent threads.
     *
     *
     *  Unless otherwise noted, passing a `null` argument to
     * a method of this class will cause a
     * [NullPointerException][java.lang.NullPointerException] to
     * be thrown.
     *
     * @see Encoder
     *
     * @since   1.8
     */
    class Decoder {
        companion object {
            /**
             * Lookup table for decoding unicode characters drawn from the
             * "Base64 Alphabet" (as specified in Table 1 of RFC 2045) into
             * their 6-bit positive integer equivalents.  Characters that
             * are not in the Base64 alphabet but fall within the bounds of
             * the array are encoded to -1.
             *
             */
            internal val fromBase64 = IntArray(256)

            /**
             * Lookup table for decoding "URL and Filename safe Base64 Alphabet"
             * as specified in Table2 of the RFC 4648.
             */
            private val fromBase64URL = IntArray(256)

            init {
                fromBase64.fill(-1)
                for (i in Encoder.toBase64.indices) fromBase64[Encoder.toBase64[i].toInt()] = i
                fromBase64['='.toInt()] = -2
            }

            init {
                fromBase64URL.fill(-1)
                for (i in Encoder.toBase64URL.indices) fromBase64URL[Encoder.toBase64URL[i].toInt()] = i
                fromBase64URL['='.toInt()] = -2
            }
        }

        /**
         * Decodes all bytes from the input byte array using the [Base64]
         * encoding scheme, writing the results into a newly-allocated output
         * byte array. The returned byte array is of the length of the resulting
         * bytes.
         *
         * @param   src
         * the byte array to decode
         *
         * @return  A newly-allocated byte array containing the decoded bytes.
         *
         * @throws  IllegalArgumentException
         * if `src` is not in valid Base64 scheme
         */
        fun decode(src: ByteArray): ByteArray {
            var dst = ByteArray(outLength(src, 0, src.size))
            val ret = decode0(src, 0, src.size, dst)
            if (ret != dst.size) {
                dst = dst.copyOf(ret)
            }
            return dst
        }


        private fun outLength(src: ByteArray, sp: Int, sl: Int): Int {
            var sp = sp
            var paddings = 0
            var len = sl - sp
            if (len == 0) return 0
            if (len < 2) {
                throw IllegalArgumentException(
                    "Input byte[] should at least have 2 bytes for base64 bytes"
                )
            }
            if (src[sl - 1].toChar() == '=') {
                paddings++
                if (src[sl - 2].toChar() == '=') paddings++
            }
            if (paddings == 0 && len and 0x3 != 0) paddings = 4 - (len and 0x3)
            return 3 * ((len + 3) / 4) - paddings
        }

        private fun decode0(src: ByteArray, sp: Int, sl: Int, dst: ByteArray): Int {
            var sp = sp
            val base64 = if (false) fromBase64URL else fromBase64
            var dp = 0
            var bits = 0
            var shiftto = 18 // pos of first byte of 4-byte atom
            while (sp < sl) {
                if (shiftto == 18 && sp + 4 < sl) {       // fast path
                    val sl0 = sp + (sl - sp and 3.inv())
                    while (sp < sl0) {
                        val b1 = base64[src[sp++].toInt() and 0xff]
                        val b2 = base64[src[sp++].toInt() and 0xff]
                        val b3 = base64[src[sp++].toInt() and 0xff]
                        val b4 = base64[src[sp++].toInt() and 0xff]
                        if (b1 or b2 or b3 or b4 < 0) {    // non base64 byte
                            sp -= 4
                            break
                        }
                        val bits0 = b1 shl 18 or (b2 shl 12) or (b3 shl 6) or b4
                        dst[dp++] = (bits0 shr 16).toByte()
                        dst[dp++] = (bits0 shr 8).toByte()
                        dst[dp++] = bits0.toByte()
                    }
                    if (sp >= sl) break
                }
                var b: Int = src[sp++].toInt() and 0xff
                if (base64[b].also { b = it } < 0) {
                    if (b == -2) {         // padding byte '='
                        // =     shiftto==18 unnecessary padding
                        // x=    shiftto==12 a dangling single x
                        // x     to be handled together with non-padding case
                        // xx=   shiftto==6&&sp==sl missing last =
                        // xx=y  shiftto==6 last is not =
                        require(
                            !(shiftto == 6 && (sp == sl || src[sp++].toChar() != '=') ||
                                    shiftto == 18)
                        ) { "Input byte array has wrong 4-byte ending unit" }
                        break
                    }
                    throw IllegalArgumentException("Illegal base64 character " + src[sp - 1].toInt().toString(16))
                }
                bits = bits or (b shl shiftto)
                shiftto -= 6
                if (shiftto < 0) {
                    dst[dp++] = (bits shr 16).toByte()
                    dst[dp++] = (bits shr 8).toByte()
                    dst[dp++] = bits.toByte()
                    shiftto = 18
                    bits = 0
                }
            }
            // reached end of byte array or hit padding '=' characters.
            when (shiftto) {
                6 -> {
                    dst[dp++] = (bits shr 16).toByte()
                }
                0 -> {
                    dst[dp++] = (bits shr 16).toByte()
                    dst[dp++] = (bits shr 8).toByte()
                }
                else -> require(shiftto != 12) {
                    // dangling single "x", incorrectly encoded.
                    "Last unit does not have enough valid bits"
                }
            }
            // anything left is invalid, if is not MIME.
            // if MIME, ignore all non-base64 character
            while (sp < sl) {
                throw IllegalArgumentException(
                    "Input byte array has incorrect ending byte at $sp"
                )
            }
            return dp
        }
    }

}

Base64Test.kt

import org.junit.jupiter.api.Assertions.assertArrayEquals
import org.junit.jupiter.api.Test
import kotlin.random.Random.Default.nextBytes
import kotlin.test.assertEquals

/**
 * This JVM specific test can verifiy the common Base64 implemenation
 */
class Base64Test {

    @Test
    fun encodeDecode() {
        (0..100).forEach { i ->
            val input = nextBytes(i * 10)

            val javaEncoded = java.util.Base64.getEncoder().encodeToString(input)
            val kotlinEncoded = String(Base64.encoder.encode(input))
            assertEquals(javaEncoded, kotlinEncoded)

            assertArrayEquals(input, Base64.decoder.decode(kotlinEncoded.encodeToByteArray()))
        }
    }

}

Thank you